Radio frequency identification (RFID) systems typically include at least one reader and a plurality of transponders, which are commonly termed credentials, cards, tags, or the like. Each transponder is an active or passive radio frequency communication device which is directly attached to or embedded in an article to be identified or otherwise characterized by the reader. Alternatively, the transponder is embedded in a portable substrate, such as a card, tag, or the like, carried by a person or an article to be identified or otherwise characterized by the reader.
An active transponder is powered up by its own internal power supply, such as an internal battery, which provides the operating power for the transponder circuitry. In contrast, a passive transponder is dependent on the reader for its power. The passive transponder typically consists of an integrated circuit (IC) chip coupled to a resonant LC circuit which has a capacitor and an inductive antenna in parallel or in series. The reader “excites” or powers up the passive transponder by transmitting excitation signals of a given frequency into the proximal space surrounding the reader. When the transponder resides in the proximal space, its inductive antenna receives the excitation signals which are converted into the operating power for the IC chip of the recipient transponder.
The powered up transponder generates transponder data signals which are in the form of electromagnetic waves embodying information such as identity data or other characterizing data stored in the memory of the IC chip. The transponder data signals are characterized by a specific carrier frequency which generally corresponds to the frequency of the excitation signals. The carrier frequency is inter alia a function of the transponder LC circuit and is often unique to the particular manufacturer of the transponder used to generate the transponder data signal. The transponder manufacturer establishes a desired carrier frequency of transponder data signals by tuning the LC circuit to a resonant frequency which corresponds to the desired carrier frequency.
The resonant frequency (and correspondingly the carrier frequency) of commercially-available passive transponders conventionally employed in RFID applications generally fall within either a low frequency range or a high frequency range. The low frequency range extends about a nominal low frequency of 125 kHz and is typically within a range of 100 to 150 kHz. In contrast, the high frequency range extends about a nominal high frequency of 13.56 MHz. Low frequency transponders are commonly termed proximity credentials and high frequency transponders are commonly termed smart credentials. It is apparent from the above that there can be significant variability in transponder frequencies even among different types of low frequency transponders or among different types of high frequency transponders.
In any case, the transponder data signals are transmitted via the transponder antenna into the proximal space surrounding the reader in which the transponder resides. The reader contains its own LC circuit having a capacitor and an inductive antenna which is tuned to essentially the same resonant frequency as the transponder LC circuit, thereby rendering the reader and transponder compatible. The reader LC circuit receives the transponder data signals and is coupled to additional reader circuitry, which enable the reader to “read” the transponder data signals (i.e., extract the data from the transponder data signals). Accordingly, contactless communication is effected between the reader and the transponder in accordance with a specific communication protocol, which is likewise often unique to the particular manufacturer of the transponder and/or reader.
The excitation signal generating and transmitting functions and the transponder data signal receiving and reading functions performed by the reader as described above define a mode of reader operation termed a “data transaction mode.” The data transaction mode further encompasses reader data signal generating and transmitting functions, wherein information stored in the reader memory or otherwise generated by the reader is communicated to the transponder. The manner in which the reader communicates information to the transponder is essentially the same or similar to the manner in which the transponder communicates information to the reader. As such, the reader data signals are characterized by essentially the same carrier frequency as the transponder data signals.
Although a reader can continuously operate in the data transaction mode, the functions of the data transaction mode typically have a relatively high power demand, which can rapidly deplete the power supply of the reader. This condition is particularly undesirable when the reader is powered by a self-contained portable power supply, such as a small disposable or rechargeable battery, which has a finite life. It is generally more power efficient to operate the reader in the data transaction mode only when a transponder is within the read range of the reader and to operate the reader in an alternate mode having a relatively lower power demand at all other times. A preferred alternate lower power mode of operation is termed a detection mode, which is commonly enabled by a ring signal generator circuit and a transponder detection circuit provided within the reader. The reader operates continuously in the detection mode except when the transponder detection circuit detects a transponder within the read range of the reader. The reader switches to the data transaction mode upon detection of a transponder, but only for a limited time sufficient to complete communication between the reader and transponder before switching back to the detection mode.
U.S. Pat. No. 6,476,708 to Johnson (the '708 patent), which is incorporated herein by reference, discloses an exemplary reader having a low power detection mode and a high power data transaction mode of operation. The reader includes a signal generator circuit, having solid-state electronics, which alternately acts as the ring signal generator circuit or an excitation signal generator circuit depending on the operating mode of the reader at any given time. The reader further includes a small portable battery power supply and the transponder detection circuit which is coupled to the signal generator circuit.
The operating principle of the detection mode is to detect a transponder within the read range of the reader by measuring changes in a response on the reader antenna. The detection mode is initiated by generating a detection pulse using the signal generator circuit and applying the detection impulse to the reader antenna. The detection impulse causes the reader antenna to transmit a ring signal into the surrounding space, which has a frequency corresponding to the resonant frequency of the tuned LC circuit of the reader. The resulting ring signal causes a predictable response on the reader antenna. Although the ring signal has insufficient to power to operate any transponders residing in the surrounding space, if a transponder having a resonant frequency at or near the resonant frequency of the reader is sufficiently proximal to the reader, the response on the reader antenna is altered in a characteristic manner. In particular, inductive coupling of the reader antenna to the nearby transponder antenna causes a change in the response on the reader antenna.
The reader employs the transponder detection circuit to detect this change in the response. In particular, the transponder detection circuit monitors the level of a designated transponder detection parameter of the response. When the transponder detection parameter reaches a predetermined threshold level, the presence of a transponder in the surrounding space is confirmed and the transponder detection circuit switches the signal generator circuit from the low power detection mode to the high power data transaction mode thereby terminating generation of the ring signals. As such, the signal generator circuit transitions to an excitation signal generator circuit, wherein the signal generator circuit draws increased electrical current from the reader power supply to generate and transmit an excitation signal which is sufficient to activate the transponder. The excitation signal is received by the transponder and powers the transponder circuitry, which in turn generates a transponder data signal for transmission to the reader. After the reader reads the received transponder data signal, the signal generator circuit switches back to the detection mode and resumes generation of the ring signals while terminating generation of the excitation signals.
Since only ring signals are transmitted by the reader during the detection mode, the reader runs at a very low duty cycle and a variable repetition rate for the duration of the detection mode. Consequently, the above-described technique enables the reader to operate with a relatively low average power consumption to avoid accelerated dissipation of the reader power supply while maintaining a rapid response time for transponder detection.
The sensitivity, and correspondingly the detection range, of the reader in the detection mode is inter alia dependent on closely matching the tuned resonant frequencies of the reader and transponder LC circuits. However, as noted above, many transponder manufacturers elect to tune their transponders to resonant frequencies which differ from those of other transponder manufacturers. As a result, many different types of transponders are commercially available to practitioners designing, implementing or managing RFID systems, wherein each transponder type is characterized by a different transponder frequency, which is termed the transponder signature. Accordingly, when a given RFID system includes transponders from one or more different transponder manufacturers, the total population of transponders in the RFID system often includes one or more different types of transponders. Therefore, it is desirable to provide a transponder detector for a reader which is capable of detecting the presence of a transponder and determining the specific type of detected transponder.
Accordingly, it is generally an object of the present invention to provide a transponder detector for detecting the presence of a transponder selected from a plurality of transponder types and determining the specific type of transponder detected. More particularly, it is an object of the present invention to determine the specific type of transponder detected by comparing the signature of the detected transponder to the signature of a known transponder type. It is another object of the present invention to provide a transponder detector which acquires analog signals resulting from the transmission of a progression of detection signals at multiple frequencies and uses the analog signals to ascertain the signature of a detected transponder. It is another object of the present invention to provide a transponder detector which acquires analog signals resulting from the transmission of a progression of detection signals at multiple frequencies to ascertain a signature of a detected transponder relating to a specific transponder frequency from among a plurality of known transponder signatures. It is a further object of the present invention to provide a transponder detector which converts the acquired analog signals to digital signals and processes the digital signals to ascertain the signature of a detected transponder. It is a further object of the present invention to provide a transponder detector which processes the digital signals using a digital signal processing algorithm to ascertain the signature of a detected transponder. It is yet a further object of the present invention to provide a transponder detector which processes the digital signals using a digital signal processing algorithm, wherein the digital signal processing algorithm is designed to protect the solid-state circuitry of the transponder detector from damage due to high radio frequency energy field strength. It is still a further object of the present invention to provide a transponder detector which initiates a data transaction mode of operation in a reader upon detecting the presence of a transponder and determining the type of detected transponder.
It is generally another object of the present invention to provide a transponder detector which can effectively operate in a detection mode at a very low power level relative to the power level of the data transaction mode of an associated reader. It is generally another object of the present invention to provide a transponder detector which can effectively operate while magnetically interfering objects other than transponders are in the proximal space of the transponder detector. These objects and others are accomplished in accordance with the invention described hereafter.